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The Loam Ranger - GPS

Dear Loam Ranger,

Santa brought me a GPS receiver. It looks great. What do I do with it?

The US Government has used a network of satellites for many decades to give its armed forces the ability to calculate their position anywhere on the Earth’s surface. The signals used to be encrypted, preventing “enemy” forces from using the system to attack US forces. But in 2000, under presidential instruction, the military removed the encryption so that civilians could benefit too. (Certain restrictions remain.)

Within 2 days, the first recreational use of GPS was instigated, when a man in Oregon, USA, hid a “treasure” and posted its latitude and longitude on the Internet. The game is today known as geocaching, and is played around the world on every continent, including Antarctica. All the player needs is a GPS receiver and sturdy shoes.

Besides treasure hunting, however, GPS has proven a boon to people in many industries, as it allow precise location of any item anywhere above ground. Surveyors routinely use it now. Robotic systems for planting, spraying or harvesting crops use it. Ships use it. Airliners use it. In-car GPS navigators are now becoming common.

Chances are you use GPS, or someone you work with does.

How does it work?

The Global Positioning System relies on 24 satellites that orbit the Earth at an altitude of 20 200 km. In fact, there are currently 31 in operation, offering redundancy in case of failure. Each satellite orbits the Earth twice a day, and the orbits are arranged so that at least six satellites are visible in the sky at any moment. Each satellite sends and receives signals indicating its position to the other satellites and to base stations on the ground. The time it takes a signal to travel, at the speed of light, indicates distance. From the signals it receives, each satellite calculates its position above the Earth to an extraordinary precision, then broadcasts that information to the ground.

Your GPS receiver is a radio receiver tuned to the microwave band. It “listens” for GPS signals, identifying different satellites by their unique call signs. With a minimum of three, and ideally four, signals, the GPS receiver can calculate its position on the Earth’s surface. It uses nanosecond differences in the arrival times of the satellite signals to mathematically compute its own position. Think back to trigonometry at school and you’ll understand how your position can be calculated from triangles.

The typical precision of a civilian GPS receiver is about 5 m. That is, you can be sure of your position within a circle of 5 m radius centred on you. This is way more precise than most maps, and is good enough to dock ships in the pitch dark.

Practical applications of GPS

A GPS receiver costs from about $200, though you can spend much more than that, especially if you want one that gives you a colour map and the location of the nearest coffee shop, and tells you in dulcet tones how to get there. Some mobile phones now have a GPS receiver built in.

Provided you have an unobstructed view of the sky—that is, no dense trees, you’re not in a tunnel and you’re not surrounded by skyscrapers—GPS works in all weathers, 24 hours a day.

Applications are unlimited:

• Surveying
• Crop dusting
• Pinpointing soil sampling sites
• Measuring heights and depths in landscaping
• Setting out orchards and vineyards
• Returning to specific sites
• Finding your way in a new place
• Verifying boundaries
• Site mapping 
• … and even treasure hunting.